Many catalyzed reactions may be accelerated by the use of ligands which bind to the catalyst and enhance its catalytic activity. Ligand acceleration of catalytic transformations is a desirable phenomenon since, in addition to increasing the rate of the reaction, the ligand may also influence the selectivity features of the reaction. (Berrisford et al. Angew. Chem. 1995, 107, 1159-1171; Berrisford et al. Angew. Chem., Int. Ed. Engl. 1995, 34, 1059-1070.)
Use of aqueous H.sub.2 O.sub.2 as the oxidant in transition metal-catalyzed epoxidations was first described by Venturello et al. (Venturello et al. J. Org. Chem. 1983, 48, 3831-3833; Venturello et al. J. Org. Chem. 1988, 53, 1553-1557; Prandi et al. Tetrahedron Lett. 1986, 27, 2617-2620.) Venturello disclosed the use of a tungstate catalyst under phase transfer conditions. This method is unemployable for less reactive olefins. However, Noyori recently introduced a solvent-free variation which is more effective. (Sato et al. J. Org. Chem. 1996, 61, 8310-8311.) However, the scope of the Noyori variation is limited by epoxide opening-problems caused by the slight acidity of the reaction milieu. This is a recurring problem encountered by most epoxidation methods in use today.
An industrial metal-catalyzed epoxidation process which uses 30% aqueous H.sub.2 O.sub.2 is disclosed by Romano et al. (Chim. Ind. (Milan) 1990, 72, 610-616; Clerici et al. J. Catal. 1993, 140, 71-83.) The method is based on the heterogeneous titanium-substituted silicalite catalyst (TS-1) developed by Enichem. The process is best employed with small, unbranched terminal olefins.
Commercial bulk production of alkyl-epoxides is achieved by oxidation of alkenes using t-butyl peroxide as an oxidant and Mo.sup.+6 as a catalyst. Supplies and pricing of t-butyl peroxide are undependable because t-butyl peroxide is a by-product of another process. Furthermore, the range of substrates that can be oxidized by this procedure to form epoxides is highly limited.
Inorganic rhenium compounds such as Re.sub.2 O.sub.7 or ReO.sub.3 are known to exhibit modest catalytic activity for H.sub.2 O.sub.2 -based oxidations (Applied Homogeneous Catalysis with Organometallic Compounds, Cornils et al. (Eds.), VCH Weinheim, 1996). Herrmann et al. disclose that the catalysis of H.sub.2 O.sub.2 -based oxidations of olefins may be significantly enhanced by the use of alkylrhenium oxide as a catalyst. (Hoechst et al. DE 3.902.357 (1989); Angew. Chem., Int. Ed. Engl. 1991, 30, 1638-1641; Herrmann et. al. J. Mol. Catal. 1994, 86, 243-266; Adam et al. Angew. Chem. 1996, 108, 78-581; Angew. Chem., Int. Ed. 1996, 35, 533-535; Boelow et al. Tetrahedron Lett. 1996, 37, 2717-2720.) Herrmann discloses that organometallic oxorhenium(VII) species are powerful epoxidation catalysts with H.sub.2 O.sub.2 as oxidant. Methyltrioxorhenium (MTO or CH.sub.3 ReO.sub.3) is a particularly active catalyst. Methyltrioxorhenium (MTO) was first prepared by Beattie and Jones. (Beattie et al. Inorg. Chem. 1979, 18, 2318-2319.)
Herrmann discloses that epoxides produced by his alkylrhenium oxide catalyzed reaction are stabilized by basic ligand additives, e.g. pyridine. However, Herrmann also discloses that these same basic ligand additives have the undesirable property of inhibiting the alkylrhenium catalysis. The addition of either tertiary nitrogen bases or pyridine (basic ligand additives) was found to suppress epoxide ring opening processes. Moreover, the stabilizing effects of these additives is characterized as occurs at the expense of a strong detrimental effect on catalyst activity. In balance, Herrmann does not recommend the use of basic ligand additives in connection with his alkylrhenium oxide catalyzed epoxidation process.
Herrmann also discloses that his alkylrhenium oxide catalyzed olefinic epoxidation reaction should be performed under anhydrous conditions. Hermann's work focuses on the use of anhydrous H.sub.2 O.sub.2 (particularly in t-BuOH) because water was detrimental, presumably due to hydrolytic epoxide ring opening. Such "anhydrous" H.sub.2 O.sub.2 solutions would probably be unsuitable for large scale applications. A more serious limitation of their process is the strong tendency for the epoxide product to be destroyed through ring opening reactions. Furthermore, the prior art teaches that an excess of olefin is needed to drive epoxidation reactions. These problems have not been overcome by Herrmann or others. (Pestovsky et al. J. Chem. Soc., Dalton Trans. 2 1995, 133-137; Al-Ajlouni et al. J. Org. Chem. 1996, 61, 3969-3976; ARCO Chemical Technology US 5.166.372 (1992)).
Tucker et al. (U.S. Pat. No. 5,618,958) disclose the use of alkylrhenium oxide catalysts with chiral ligands, including imine ligands, for catalyzing asymmetric epoxidations of olefins using pressurized oxygen and organic solvents. However, Tucker does not disclose the use of nitrogenous aromatic heterocycles as accelerants in connection with organorhenium oxide catalyzed epoxidations of olefin. Furthermore, Tucker does not disclose the use of aqueous solvents or of hydrogen peroxide as an oxidant.
What is needed is a process which overcomes the limitations of the rhenium oxide catalyzed epoxidation which are undesired hydrolytic epoxide ring opening, the requirement of excess olefinic substrate, the requirement of anhydrous conditions, the use of specific olefinic substrates, low yields, and the inhibition of the rhenium oxide catalyst by the basic ligand additives.
Furthermore, an accelerated epoxidation process is needed which employs commercially available accelerants, commercial grade hydrogen peroxide as an oxidant and a commercially available organorhenium oxide as a catalyst in mild aqueous solvent conditions and at ambient temperatures.